The invention relates to a probe for quantitatively measuring volatile components of a liquid comprising a measurement gas space, said measurement gas space being separated from the liquid by a permeation membrane being permeable for the volatile component, however non-permeable for liquid, and comprising a selective gas detection system arranged in the measurement gas space for a defined volatile component.
For monitoring, controlling and/or regulating certain chemical, biotechnological, food-processing, or pharmaceutical processes, it is necessary to determine and monitor on-line, in-line or off-line the alcohol content of a solution or suspension. Further, it is regularly desirable to simultaneously detect and monitor other volatile components of the solution or suspension, such as CO2. An example of an economically important process is beer brewing.
Volatile components of a liquid may be, in addition to C1 to C6 alkyl monoalcohols, C1 to C8 hydrocarbons, C1 to C6 alkyl aldehydes, C1 to C6 alkyl ketones, C1 to C6 alkyl carboxylic acids, CO2 and O2. In the beer brewing technology, in particular C1 to C3 alkyl monoalcohols, in particular ethanol, CO2, and/or O2 have to be determined and monitored.
For instance from the documents EP 0 174 417 E1 and DE-199 59 271 A1 there are known in the art probes by means of which the alcohol content of a liquid-can be determined on-line or in-line. In the in so far known measures, alcohol determination is made by means of a solid state detector. A solid state detector typically comprises a semiconductor element, for instance on the basis tin oxide, at the surface of which alcohol is reacted, thus an electrical signal being generated that is fed to an evaluation circuitry. The in so far known technology is well proven. There is however a requirement for the determination of high alcohol contents, i.e. in the range from 2% v/v (referred to the liquid) to 20% v/v. With such high alcohol concentrations, measurements with solid-state detectors are however inaccurate or even impossible.
From the document DE 100 25 686 A1 it is known in the art of the beer brewing technology to simultaneously determine the alcohol content and the index of refraction of a liquid. In this approach, again a tin oxide sensor is used, with the above drawback. Further, this prior art sensor operates without a carrier gas with the consequence of a very high time constant.
The invention is based on the technical object to specify a probe, by means of which high alcohol concentrations can reliably be determined
For achieving the above technical object, the invention teaches that the gas detection system comprises an IR light source, an IR filter, and an IR-sensitive light sensor, light falling exclusively through the IR filter on the light sensor, and that the IR filter comprises a transmission window in the range of an alcohol-specific absorption band.
The term of the IR-sensitive light sensor comprises, in addition to photosensors, also thermal sensors, for instance pyroelectric sensors. An IR light source transmits IR light in the range of the transmission window. A transmission window is the property of a component to only let pass a wavelength range definable by the width at half-maximum (referred to maximum transmission) about a defined center wavelength. The term wavelength will in the following always refer to the center wavelength. Transmission windows will not overlap, if the widths at half-maximum do not overlap.
The invention is first of all based on the finding that high alcohol contents can also be measured with the usual permeation membranes. Subsequently thereto, it was found that by means of an IR absorption measurement a linear or easily linearizable (for instance by calibration with a transfer function obtained by measurement of a reference solution or of several reference solutions) signal is obtained, at concentrations of up to 20% v/v and higher in the liquid. Of particular importance is here that not only an increase of a carrier gas flow through the measurement gas space is not necessary anymore (with this measure, semiconductor detectors can also be used for higher alcohol contents), rather a carrier gas flow may completely be left out. This permits the use of mobile portable probes, since a carrier gas source needs not be taken along. Prior to such a measurement or between two measurements, a rinsing only of the measurement gas space needs be performed, for instance by ambient air.
The transmission window suitably is provided at a wavelength of 1.0 to 3.0 xcexcm, preferably 1.5 to 2.5 xcexcm, in particular 2.0 to 2.2 xcexcm.
The IR filter can in principle be located at different places. If the filter is arranged immediately at the IR light source, then only light having the wavelengths of the transmission window will pass the measurement gas space or parts thereof. Then the light sensor needs not be wavelength-selective. It is however preferred that the IR filter is arranged immediately in front of the light sensor. Then, light having wavelengths of the complete IR range will pass the measurement gas space or parts thereof, whereas the combination of light sensor and IR filter forms a wavelength-sensitive light sensor. In any case it is recommended that the light sensor is protected or shielded from foreign light.
For stationary applications, for instance the on-line or in-line measurement in a flowing liquid, with the permeation membrane being immersed in the liquid, it is preferred that the measurement gas space is passed by a preferably continuous carrier gas flow, i.e. the alcohol molecules being convectively transported through the measurement gas space. Carrier gases may be air, nitrogen, etc. By adjusting a defined carrier gas flow, a very high accuracy is achieved. The liquid may pass by the permeation membrane (in-line measurements) or rest (laboratory measurement of a sample).
For mobile or portable embodiments of the invention it is however preferred that no carrier gas is used. The only measure taken is that prior to a measurement the measurement gas space is rinsed with for instance air. A small pump may be provided therefor, said pump being connected to a rinse gas inlet. Suitably a rinse gas outlet is also provided. After rinsing, the rinse gas inlet and the rinse gas outlet are closed. The permeation membrane is then immersed into a taken liquid sample, and that so long until a constant measurement value is achieved, i.e. a balance between the alcohol content of the sample and the alcohol content of the measurement gas space exists. The transportation of the alcohol molecules in the gas phase thus takes place in a purely diffusive manner.
In an embodiment of the invention having an independent importance, a second light sensor and a second IR filter, preferably immediately in front of the second light sensor, are provided in the measurement gas space, the second IR filter having a transmission window in the range of a CO2-specific absorption band, and the transmission windows of the first IR filter and of the second IR filter do not overlap. In this embodiment, CO2 and alcohol can be measured with a single probe. In the case of the carrier gas-free operation and rinsing with air, there takes place of course a zero measurement of CO2, the value of which is subtracted from the measured value for the determination of the CO2 content of the liquid. Instead of CO2 or in addition thereto, the probe can be configured for the simultaneous measurement of further volatile components by addition of light sensors and filters with corresponding transmission windows.
In the case of the CO2 measurement it is preferred that the transmission window of the second IR filter is provided at a wavelength from 3.0 to 5.0 xcexcm, preferably from 4.0 to 5.0 xcexcm, in particular from 4.1 to 4.5 xcexcm.
The measurement of further components, in particular CO2 in addition to alcohol, may be provided in different ways. The measurement gas space may comprise: i) a permeation gas space, ii) a first IR measurement cuvette with a first IR light source, a first IR filter, and a first light sensor, and iii) a second IR measurement cuvette with a second IR light source, a second IR filter, and a second light sensor, wherein the permeation gas space, the first IR measurement cuvette, and the second IR measurement cuvette are connected to one another in a gas diffusive or convective manner, and wherein the transmission windows of the first IR filter and the second IR filter do not overlap one another and are provided in a range of an alcohol-specific and a CO2-specific absorption band. Alternatively, the measurement gas space may comprise: i) a permeation gas space, and ii) a single IR measurement cuvette with an IR light source, a first IR filter, a second IR filter, and at least one light sensor, wherein the permeation gas space and the IR measurement cuvette are connected to one another in a gas-diffusive or convective manner, and wherein the transmission windows of the first IR filter and the second IR filter do not overlap one another and are provided in a range of an alcohol-specific and a CO2-specific absorption band. In the latter case, a particularly simple design is achieved.
A further simplification of the design and reduction of the dimensions and weight is obtained, if the probe comprises a probe finger with a probe finger space separated by the permeation membrane from the liquid and a probe body carrying the probe finger, the at least one IR measurement cuvette being arranged in the probe body and being connected with the probe finger space in a diffusive or convective manner. Alternatively, the probe may comprise a probe finger with a probe finger space separated by the permeation membrane from the liquid and a probe body carrying the probe finger with a probe body gas space, and at least one IR measurement cuvette carried by the probe body, the probe finger space, the probe body space, and the IR cuvette being connected to one another in a diffusive or convective manner.
In the measurement gas space further detectors for the most various volatile components may be provided. Thus, it is also possible to provide an alcohol-specific solid-state gas sensor in the measurement gas space, in particular in the probe body space, for the measurement of low alcohol concentrations (up to 2% v/v).
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.